Method of electropolishing



i Sep Filed Feb. 8, 1947 S. E. EATON METHOD OF' ELECTROPOLISHING 2 SHEETS-SHEET l if@ ,a

l fnventan A js.

2 SHEETS- SHEET 2 fnl/anto?? A Ms.

CURRENT DENSTY MMM {gba-0W Samuel E Eaton S. E. EATON METHOD OF' ELECTROPOLISHING CURRENT DENSITY Sept. 9, 1952 Filed Feb. 8, 1947 Patented Sept. 9, 1952 UNITED STATES PATENT OFFICE METHOD OF ELECTROPOLISHING Samuel E. Eaton, Lexington, Mass., assignor, by mesne assignments, to Oneida, Ltd., Oneida, N. Y., a corporation of New York l2 Claims.

This invention relates to a method of electropolishing metal surfaces.

In my copending application Serial No. 474,437, filed February 2, 1943, now U. S. Patent No.

2,416,294 (the disclosure of which is hereby incorporated by reference and of which this application is accordingly a continuation-in-part) a method of polishing metals is described wherein the metal surface to be polished is made the anode in an aqueous electrolyte containing a soluble cyanide and subjected to an electrical potential difference (i. e., the terminal voltage measured between the electrodes of the electrolytic cell) which is between that at which the metal surface manifests a polishing effect and that at which a spontaneous flashing phenomenon takes place or permanent; film formation occurs. The potential difference is then maintained substantially constant until the metal surface acquires a polished surface, for example in a quiescent bath. This result may be further promoted by agitation of the anode or of the electrolyte, without turbulence, and by preventing too close an approach of the anode to other solids.

It is now discovered as disclosed in my copending application Serial No. '727,452 that while a controlled and constant potential difference across the electrolytic cell is a useful and desirable method for electropolishing of the metal anode, when commercial application of the method is made the practical problem of accurately maintaining the potential difference at a desired point is sometimes presented.

As disclosed in my said copending application Serial No. '727,452 it is found that a potential difference across the cell, which is positively uctuated between predetermined limits of voltage and current density and in predetermined periods of fluctuation, results in an improved commercial method yielding reproducible polished surfaces comparable to those obtained by the method of holding the potential difference constant. The factor that is thus left to be closely controlled is that of time which, for commercial practice, is less costly than accurate Voltage control.

When a metal surface-which may, for example, be a silver surface which has been formed by electroplating, in which operation it constituted the cathode-is made the anode of an electrolytic cell, containing an alkaline cyanide in an aqueous electrolyte, and the imposed potential difference is varied from Zero upward, various changes in the appearance of the surface may be observed.

Considering the surface of only a very small area of the above anode when of silver, for example, the following sequence of changes appears to take place. As the voltage is increased, the current density increases and the surface is etched to a light grey color which remains up F 2 to a point called herein the polarization. point. This point marks a change in the relationship between further increase in potential difference with respect to increasing current densitylin that the current density no longer continues to increase at the same rate. It may increase, but at a slower rate, or it may even not increase at all. If maintained at potential differences slightly above the polarization point the etched appearance of the surface changes to one of specular brightness and covering the surface is a light brown coloration which may be hardly visible. If the potential difference is increased, and the polished area is closely observed, the brown coloration will be seen to become darker, though if the current is cut olic or the anode removed from solution such discoloration disappearsleaviing the surface polished. As the potential difference is raised still further, a peculiarly unstable condition is usually encountered which is termed spontaneous flashing. In this region the brown discoloration noted above repeatedly and spontaneously fades and returns giving the appearancey of flashing. The appearance when faded is one of light brown or grey. Above this potential difference there is a gradual change to a light grey coloration which may be due to an etching effect; and upon increasing the Vpotential difference, it becomes darker. These later colorations which are formed at potential differences above the flashing point, as contrasted with the brown coloration discussed previously, are permanent-and especially that coloration which occurs when the anode is maintained at a suf- Alicient potential difference and for a suiiicie'nt time tending to produce the dark grey coloration. By permanent is meant that the coloration does not readily or completely disappear when the current is cut on or the anode removed from the cell.

The above discussion relates to a givenvery small area of a silver anode surface. In practice it is observed that corners and sharp edgesof the anode become polarized-first because of higher current density at these points. Upon gradually increasing the potential dilerence the brightening and other effects on these parts as described above appear to precede Such changes on other parts which are of lower current density. Thus in the case of spoons the edges would leX- hibit the sequence of change first and the concave center of the bowl would pass through the sequence and hence receive its polish last. y

The observed relationships of potential difference to current density with respect to the elec-- trodes and the cell as a whole are therefore averages of the varying effects taking place throughout the entire surface of the anode. This illus trates why if voltage alone is controlled and constant, the potential difference at Whichfall The exact mechanism of thev polishing phe nomenon is not understood, in its entirety, but

appears to relate to the unequalldissolution 'of' silver from protuberances as comparedftolde';

pressions in the surface, and to the continualV formation of insoluble cyanides ofthe metalto be polished in the depression and the formation of soluble cyanides on the protuberances;

, But by Whatever specific mechanism the polishing operates'this is the result, that the anode surface progressively develops Ia dependable and reproducible polish by theY imposition of-'a positively controlled and fluctuated potentialdifference thereon as above described, as contrasted with an unreproducible surface if uncontrolled voltages or current densities are allowed, as in the-spontaneous flashing phenomenon.

Agitation ofthe electrolyte (preferably without turbulence of the electrolyte) appears to lower the resistance (increase the conductivity) of'the system, causing a higher vcurrent density at -a given potential difference, and effectively promotes the polishing action ofthe cell.

The' processl may be carried out by varying thievoltage in any of a number of Ways. There are, however, certain limitations which should be observed. Excessivetime either in the region belowfthepolari'zation point or in therregion of permanent discoloration is toy be avoided. While' the time period or periods of fluctuation maybe varied, no advantage is ob'tainedby too long a period of uctuation. Short periods of uctuation are sometimes more effective to develop the polishing action and the polished surface on the anode most satisfactorily.

The prin-cipal advantage of the present process is'the `elimination of the necessity for close controlofa constantly maintained Voltage. Moreover the positively controlled fluctuation of the voltage into the'regionsl above and/ or below the polishing range are notonly permissible but found to be advantageous because easier from the-standpoint of practical operations.- It has been found that best polishing results are obtained'if the voltage is not Within such regions for a'v prolonged or' dominant proportion ofthe time of each fluctuation period but is varied substantially all the operating time and predominantly or completely Within the polishing range:

Typical'examples were carried out `in Which the `potential difference was gradually increased and thevalues of the voltage and current density werel obtained and the 'characteristic qualities of the anode surface'were observed. The results are plotted in the graphs of the accompanying iigures showing typical'A relationships of voltage and'ampera'ge for the variousmetals to which the'jinvention isapplicable and successive lregions ofincreasing-voltage thereon are labelled A', B and C.

Fig. 1 presents the results obtained 'with a silver cyanide electrolyte containing 0.3 molal silver and about' 0.,29 molal free cyanide (above that required-to form vKAg (CN) 2 With the silver `present) andOLl molal K2CO3, by imposing a gradually increasing potential difference betweenV the anode andcathodes in a quiescent bath;

Fig. 2 shows results obtained with silver in' an electrolyte containing 5% KCN (about molal) with no appreciable silver cyanide or potassium carbonate, in a .quiescent bath;

Fig. 3 showsresults for'the'sameA 5% KCN bath but here the silver anode is agitated in a continuous circular path at a linear speed of 14 feet per minute, with the results shown in the graph to the right, while the results plotted in the graph to the left are quiescent conditions ofthe electrolytecbetween the electrodes and of theanolyte and'anode specifically;

Figs. 4 to'8inclusive contain graphs of results obtained similarly to those described in respect of Fig'. 3, with bronze, copper, cadmium, zinc and nickel silver respectively.

Referring to `lii'g. l, inzparticulartrelating to silver, region A representsfthe lowlvoltage. and low currentdensity .region in which.` etching :predominates, region B is the polishing region,.and regionC, the flashing and/or permanent-coloration region. As .previously explained, though these' regions. Vare' sharply separated on'. the graphs-in generalj-the eiects in one maypersist into .the` near parts of the adjacent regions.'

Regions B andClof'fgraphl have beensubdivided typically into` subdivisions B1, Bzzand'in-to C1, C2 andl C3, respectively; The upper region of permanent.color-formation C'shows threeslightly different eiectsrin the'sub'divisions. labelled C1, C2 and :03; respectively. In C3; whilethe 'current is on,A av deiinitely'k darkgrey coloration gradually forms `which is:` permanent and" may become physicallyY separable' from the. solid metalsurface.-` If'voltageais appl-iedisuddenly in this-,region,` a period .of-timeg sometimes: alfew' seconds, is required before-fthe coloration becomes permanent; The surface. turns` a dark greyfwhen first Withdrawn fromlthe bath. Part'offthe. dark grey iilm turnstoi aligh't'grey and-,the silver surface appears to be etched. underl the microscope but part of` thedarlr.v grey nlmj remains dark and Withstandsconsiderable abrasion and withstands at least a few'y minutes contact with nitric'a-cid'. This ydarlipermanent.iilrn can be scratchedoiofth'e surfacewitl' a probe' and underA the` microscope appearsv to Abe ablack 'precipitate--probabl-y` of' silver oxide;

In C2 a permanent coloration'isformed after ashort` time, which is grey to dark grey and which does4 not'.readily'redissolve or disappear upon sharp reduction .of' the kpotential diiTerence or cutting off' of the current altogether. The lighter grey sometimes appears to'be a'ne'etching ofi the surface.` Italso represents the preferred upper limit furthe4 positively imposed iiuctuations: of potential difference for polishing purposes'to avoid" the tendencies of permanent' iilm formation.-

In ,C1 the anode'mayupon continued maintenance f ofil the potential difference undergo the spontaneous a'shring phenomenon over the Whole surfaceor-parts'o it'and with very'rapi'd alternationsto very slow alternations, but without accurate: prediction, duplication orcontrol. flashing' vmay be arrested'by, for' example, shutting` oithecurrent' or by increasing the1potential differenceup intoorabove'th'e'regions-C2 or Cgor down into `region B2, B1 or'A;

The electrical power source in the particularY instances above Yplotted had a rated capacity'of 25 amperes, 'volts D. C. The voltage and amperage simultaneously uctuated with the spontaneous flashing eiect. Thus as the brown discoloration'fades, the current rises and the voltage falls.- Then the coloration reappears, thecurrent falls and the voltage rises. This is what might be expected if the coloration were an' insulating iilm. This cycle of changesl is represented in Fig. 1 by the sloping dotted lines of region C1; each dotted line schematically represents the change` in the voltage-amperage rela ticnship during the flashing cycle.` Each line corresponds to adifierent setting of the applied voltage source.

In-Zone B, two polishing eifectsare noted and labelled B1 and B2. In subdivision B2 the` entire anode develops a uniform polish. I A brown` coloration covers the surface. This colorationis dark brown at the higher voltages of region B2 and lighter brown at lower voltages. If the cur rent is loweredor the anode removed from the solution this coloration disappears to leave the surface polished. l

The subdivision B1 is generally that range o potential differences in which the anode surface shows tendencies to acquire polishing preferentially at the margins or corners while the center is usually at lower current density andy thus is less polished than the edges.

The other graphs showsimilar regions A, B and C which have the same general characteristics as those described with reference to Fig. 1. The ashing phenomenon was not observed however and therefore does not appear in the graphs.

The boundaries between different regions are not sharp because the eifect in one region graduates somewhat into that of the next, as previously mentioned. i

Polishing operations were carried out on silver anodes in electrolytes or compositions, as given in the legends` indicated uponthe accompanying graphs, with various ranges of imposed iiuctuations of potentialdifference, as indicated below.

Fig. 2 illustrates typical voltage-amperage relationships for a silver anode in a 5% KCN bath i with no agitation. A polish was obtained b y'applying a voltage of 3.5 for 5 seconds, rapidly raising to 3.7 volts and holding there for 5 seconds, thenrapidly lowering to 3.5 volts for 5 seconds, etc. The anode showed a brown discoloration when in the solution and this discoloration varied in intensity las the voltage varied. Similar results were obtained in an identical experiment which was like the above except that the time at 3.7 volts was seconds instead of 5; the time at 3.5 volts was 5 seconds.

Fig. 3 illustrates voltage amperage relationship for a silver anode in a 5% KCN bath, the anode being agitated in a continuous circular path at a rate of 14 feet per minute. A voltage of 2.7 was applied for 5 seconds and raisedrapidly to 4.6 for 5 seconds, then lowered back to 2.7 for 5 seconds, etc., and this cycle repeated Vfor a total of 2 minutes. The anode appeared grey when at 2.7 volts and brown when at 4.6 volts. The anode developed a polish which was better when withdrawn after the 4.6 volt part of the cycle. When the cycles were lengthened to 145 seconds, `the same results were obtained except that the polish was superior. i

Results indicate that preferred polishing effect is obtained if a substantial part or all of the cycle of voltage and/or current density includes opera tion at a voltage above the polarization point and below the point at which permanent discolorations would eventually be formed. As pointed out above a short period of time is required, after applying a voltage suflicient to cause eventual permanent iilm formation, before a permanent iilm is actually obtained. It appears that during this short interval, enhanced polishing is obtained because of increased dissolving action on the highest protuberances. In any case, during the remaining part of the cycle, which includes operation at voltages outside this region, the voltage preferably should not be maintained thus for a period of time sufficient for formation of the permanent discoloration, etching, or spontaneous fiashing. As in my previous application, Serial No, 474,437, the region of permanent discoloration is that voltage region above (or below) the polishing region which produces a discoloration on the metal surface, which persists when the metal is withdrawn from the bath, as contrasted with the discoloration, either brown or sometimes grey in the case of silver, which disappears almost instantly when the work is withdrawn from the bath or the current is cut off or varied, to leave a polished or slightly milky surface.

Fig. 5 shows relationships for copper in 5% `KCN A voltage of 1.8 was applied for l0 seconds,

quickly raised to 2.8 for l0 seconds, lowered to 1.8 for l() seconds, etc., for a total of 2 minutes, and the polishing eifect obtained. These voltages correspond to the upper part of region A and B, respectively. In another run, a voltage of 2.8 volts was applied for 5 seconds and then cut oif completely for 5 seconds, reapplied for 5 seconds, etc. for a total of 2 minutes and a polishing eiect was obtained. C

It may therefore be pointed out `from the figures and foregoing description of resultsthat by positively iiuctuating the potential difference across an electrolytic cell in which .Ithe anode `surfaces are of the metal to be polished andthe electrolyte contains a soluble cyanide ".(whichz is also a solvent of the metal) either continuously or abruptly, and either of equal or of predetermined degrees of variationof the cycle,` but including preferably for a substantial partof the cycle a range of values of potential difference above that at which a tendency to polishing is observed and below that at which permanent discoloration occurs, the surface of the `anode is developed with a light-reflective quality and mirror characteristics. The period of such iluctuations may also vary.

In addition to the metals specifically referred to above, the present invention is also applicable to alloys consisting of the metals silver, copper and cadmium, and also to surfaces of brass. d

Supplemental to the foregoing observations and discoveries, with respect to the anodic polishing of metals by imposing a uctuating potential difference upon the cell (either with continuous agitation of the electrolyte or in a quiescent bath), which is disclosed'and claimed in a co-pending application led February 8, 1947, `Serial No. 727,452, it is now further found that by imposing a fluctuating agitation upon the electrolyte, especially between the electrodes or in the anolyte surrounding the anode surface to be polished, or agitating the anode surfacaor both, certain advantages are also obtained.

`In this aspect of the invention, voltage-current density relationships were obtained with anodes of different metals in a 5% ECN bath under conditions of no agitation and also with agitation by rotation at a constant circular speed. The results of these experiments are plotted in the accompanying figures showing typical relationship of voltage and currentdensity for the various metals to which the invention is applicable, as above described. i

A series of experiments is described below wherein the polishing effect on thev anode caused A.by-changes in rates of-,agitation of the anode with respect to the electrolyte has been determined.

The vgeneralprocedure Yof these experiments was `as follows:

The anode was placed in the bath and conditions of voltage,' current density, andagitation rateadjustedto one set of conditions, the polish- 'ing region for instance. rEhen the agitation rate was uctuated (which caused fluctuating changes `in anodic conditionsaccordingly) such that for a substantialpart ofthe time' the operation was in the polishing yregion as` above set forth.

`It was alsonoted-that'a period of time, from a fraction of'a second ,to several minutes, is usually required for conditions to reach equilibrium after Veach change of agitation.V This phenomenon is more marked whenagitation lrate is decreased than when agitation is increased. During the period of time immediately after agitation rate is decreased and conditions of permanent coloration are approached enhanced polishing action may be observed, thus operation for short periods in this region in which permanent colorations would be rformed was found permissible and in some cases was preferred, providing the time was kept short enough to avoid the formation of damaging permanent colorations.

A typical polishing procedure with uctuating agitation follows.

A pure silver anode was placed in the bath of KCN, and rotated at a surface speed of 14 feet per minute with a. potential difference oi' 4.0. volts resulting in an anodic current density of' 58arnps. perk sq. ft., corresponding to l in the polishing region of the right-hand graph on Fig. 3, for seconds. The agitation was then stopped for 15 seconds andthe voltage and current density wereobserved consequently to approach those values indicated by 2 on the left-hand graph of Figure 3 in the permanent nlm region. Alternate agitation and no agitation cycles of 15 seconds each were maintained for 2 minutes after which vthe anode was removed, rinsed and dried. The

anode was observed to be polished an estimated `one-half perfect mirror.

Another experiment in which the same rate of agitation, voltage and current values obtained, but the timing was different. The silver anode was agitated for 14 seconds, held stationary for 1 second and this 4cycle continued for 2 minutes. The anode was observed to be polished about one-half mirror.

The same anode was a'lso alternately rotated at 14 ft. per minute for 5 seconds at a voltage of 2.7 volts, ycurrent of 39 amps. per sq. tt. corresponding to 3 on the right-hand graph of Fig. 3 in the etching region, then held stationary for seconds during whic-h time the voltage `and current approached those indicated by 4 on the left-hand graph of Fig. 3 and repeated for 2 minutes. The anode was then found to be lustrous but not as lustrous as the foregoing runs. Y

Fig. 4' presents a graph of the conditions and results obtained with an anode surface of commercial bronze. The electrolyte was subjected to uctuations `oi agitation of 15 seconds with agitation and 15 seconds without agitation, and 14 second-s with agi-tation and 1 second without agit-ation for 2 minutes. Thereupon lthe potential diiference and current density varied between points l and 2 and polish of approximately 1A? mirror was obtained in each case over '8 two-thirds tov three-quarters ofthe anode surface.

By subjecting the electrolyte to fluctuations of 5 seconds of agitation `and 25 vseconds without agit-ation for 2 minutes, polishing was obtained, which wasbetter on the Corners of the anode and the potential difference and current density varied between the points 3 and A.'

Fig. 5 shows typical curves of voltage vs. current density for a copper anode. A polishing effect was observed after 2 minutes of alternate Iagitation (15 second rotation and 15 second stationary) between points l and? on the righthand and left-hand graph l, respectively of Fig. 5. 14 second rotation alternated with 1 second stationary in the same region for 2 minutes produced a better polish, y

A polis-hing eiect was also obtained .whenthe anode was alternately agitated for 5 seconds and held stationary for 25 seconds at points V3 and 4 of Fig. 5 for copper. y

Typical curves for a cadmium anode are shown on Fig. 6. Cycles of l5 seconds rotating 15 seconds stationary for 2 minutes total, and also 14 seconds rotation, 1 second stationary for 2 minutes gave potential differences and current densities alternating between points and 2. Periods of 5 seconds rotation and 25 seconds stationary at 3 and 4 produced aithird to quarter mirror polish on the` cadmium anode.

Fig. '7 shows curves typical of cold rolled Zinc. Alternate rotation and then stationary cycles of 15-15 and 14-1 seconds between points I and 2 and 5-25 seconds between 3, and `fl, produced a luster on the zinc anode.

Fig. 8 shows Ycurves for nickel-,silver alloy. The slanting lines on the stationary curves are representative of spontaneous flashing observed. Plolishes of 1/3 to ,1/2Y mirror were obtained when 1a piece of nickel silver alloy was alternately rotated and stopped for 2 minutes in cycles of l5 seconds rotation, 15 seconds stopped, and 14 seconds rotation, l second stopped, at points indicated by l and 2 on the char-t. A polishing eiect which Ywas better on the corners of the anode was noted when rotated at a voltage represented by point 3 of Figure 8, 5 seconds and stopped (point Il) for 25 seconds alternately for 2 minutes total time.

The specific cycle is not a critical factor. It is to be understood that other variations in agitation, for example cycling between 28 feet per minute and 14 per minute, agi-tation would produce variations in voltage` in a similar manner and anode polishing would be obtained.

In the adaptation of the invention for various purposes and under specific conditions of `equipment and operation, numerous combinations will present themselves from the above disclosure which Athe invention is especially effective in accommodating.

For example, if it is desired to conduct the pro-cess so that it will draw a substantially constant load from the source of electric current, this may be very readily done, and at the same time serve the purposes of the present procedure very advantageously. Thus, the potential difference may be divided or the current may be divided between two or more cells of substantially, equal resistance under analogous conditions of operation by connecting them in series or parallel and interposingan alternating or `oscillating switch between .them by which the potential difference and/or current through one cell is reduced (and by the same degree) as the potential dierence and/or current through the other cell is increased-and vice Vversa,1-sulost-antiallycomplementary toeach other at all times.

A similar adjustment 'of the powerY requirements may be eiiected by fluctuating the potential difference and/or current through the cell `and the potential `difieren-ce and/or current;` through' the motor driving the agitating mech-4 anism and thus operating mentary synchronism.` n

Equilibrium in the etching, `polishing or `permanentcoloration regionas dis-cussed above, is that condition which, if it were maintained without change, would develop a gradualpe-rmanent etching, polishing or permanent coloration respectively of the metal anode surface.

As indicated above and exemplified in a number of instances described, iiuctuations in the agitation of the electrolyte (or of the anode with respect `*to the electrolyte) include periods of quiescence, aswell as variation-sin the degree of agitation imposed upon Ithe electrolyte or the anode with respect t-o the electrolyte;

Likewise, it may be emphasized from the fore-l going disclosure and data that while changing all three in complethe rate of agitation, there-by -to shift the po tential difference-current density values withinthe polishingregion, is effective to promote polishing and enhance the degree of` luster obtained on the anode surface, a wider degree oi fluctuation in the rate of agitation causing a correspondingly greater shiftin, the potential difference-current ydensity values and hence carry them outside of the polishing region into the other regi-on designated, `will still further enhance the polishing effect and results of the process on the anode surf-ace,

Iclaim: y A

1. Method for the anodic polishing of silver, copper, cadmium, :alloys `'consisting of these metals, brass, zinc and nickel silvergin an electrolytic cell containing a cyanide bath in which the rate of agitation of the anode with respect side said polishing region for a period of time, short enough `to, avoid the formation of `a per# marient etch or a permanent color-ation, and repeating said cycle until at lea-st a' portion of the anode acquires a polish. v

2, Method `for the anodic polishing oi silver, copper, cadmium, alloys consisting of these metals, brass, Zinc and nickel silver in an electrolytic cell containing a cyanide bath in which the rate of agitation of the anode with respect to the electrolyte aiects the potential differencecurrent density curve and the location of the equilibrium etching, polishing` and permanent coloration regions with respect to said curve, said method comprising the stepsof setting up an initial agitation of such metal with respect to the electrolyte, applying a potential difference between the anode and the cathode, said potential difference 1ceing in one of said regions outside said equilibrium polishing region for a period 10 of time short enough to avoid the formation of an etch or a permanent coloration, cyclically changing said rate of agitation to shift the resulting potential difference-current density values to a point within saidl equilibrium polishing region and repeating said cycle until at least a portion of the anode acquires a polish.

3. Method for the anodic polishing of silver in an electrolytic cell containing a cyanide bath in which the rate of agitation of the anode with respect to the electrolyte affects the potential difference-current density curve and the location of the `equilibrium etching, polishing and permanent coloration regions with respect to said curve, said method comprising the steps of setting up an initial agitation of such metal with respect Vto `the electrolyte, applying a potential difference between the anode and the cathode, said potential difference being in the equilibrium polishing region, cyclically changing said rate of agitation to shift the resulting `potential difference-current density values' to a DOnt outside said polishing region for a period of time short enough to avoid the formation of a permanent etch or a `permanent coloration, and repeating said cycle until at least a portion of the anode acquires a polish.

4. Method for the anodic polishing of silver in an `electrolytic cell containing a cyanide bath in which the rate of agitation of the anode with respect to the electrolyte affects the potential difference-current density curve and the location of the equilibrium etching, polishing and permanent coloration regions with respect yto said curve, said method comprising the steps of` set ting up aninitial agitation of such metal with respect to the electrolyte, applying a potential` difference between the anode and the cathode, said potential difference being in one of said regions outside said equilibrium polishing :region for a period of time` shortenough to avoid the formation of an etch or a permanent coloration, cyclically changing said rate of agitation to shift the resulting potential diiierencefcurrent density values to a point within said equilibrium polishing rcgion'and repeating said cycle `until at least a portion of the anode acquires a polish.

5, Methodfor the anodic polishing'of copper in an eiectrolytic cell containing a cyanide bath in which therate of agitation of the anode with respect to the electrolyte aiiects the :potential diilerence-current density curve and the location of the equilibrium etching, polishing and permanent coloration `regions with respect to said curve,` saiclrnethod` comprising the steps` of setting up an initial agitation o1" such metal with` respect to the electrolyte, applying a potential diilerence'between the anode and the cathode, saidA potential difference being in the equilibrium polishing region, cyclically changing said rate of agitation to shift the resultingpotential difierencercurrent density values to a point outside said polishing region for a period oi time short enough to avoid the formation of aper-V manent coloration regions with respect to said curve, said method comprising the steps onset forV a period of time short enough to avoid the formation of an etch or a permanent coloration, cyclically changing said rate of agitation to shift the resulting potential difference-current density values to a point within said equilibrium polishv ing region and repeating said cycle until at least a' portion of the anode acquires a polish.

7. Method for the anodic polishing of an alloy consisting of silver and copper in an electrolytic cell containing a cyanide bath in which the rate of agitation of the anode with respect to the electrolyte affects the potential difference-current density curve and the location of the equilibriuin etching, polishing and permanent colorationregions with respect to said curve, said method comprising the steps of setting up an initialy agitation of such metal with respect to the electrolyte, applying a potential difference between the anode and the cathode, said potential difference being in the equilibrium polishing region, cyclically changing said rate of agitation to shift the resulting potential differencecurrent density values to a point outside said polishing region for a period of time short enough to avoid the formation of a permanent etch or a permanent coloration, and repeating said cycle until at least a portion of the anode acquires a polish.

8. Method for the anodic polishing of an a1- loy consisting of silver and copper in an electrolytic cell containing a cyanide bath in which the rate of agitation of the anode with respect to the electrolyte affects the potential differencecurrent density curve and the location of the equilibrium etching, polishing and permanent coloration regions with respect to said curve, said method comprising the steps of setting up an initial agitation of such metal with respect to the electrolyte, applying a potential difference between the anode and the cathode, said potential difference being in one of said regions outside said equilibrium polishing region for a period of time short enough to avoid the formation of4 an etch or a permanent coloration, cyclically changing said rate of agitation to shift the resulting potential difference-current density values to a point Within said equilibrium polishing region and repeating said cycle until at least a portion of the anode acquires a polish.

9. Method for the anodic polishing of brass in an electrolytic cell containing a cyanide bath in which the rate of agitation of the anode with respect to the electrolyte affects the potential difference-current density curve and the location of the equilibrium etching, polishing and permanent coloration regions with respect to said curve, said method comprising the steps of setting up an initial agitation of such metal with respect to the electrolyte, applying a potential difference between the anode and the cathode, said potential dierence being in the equilibrium polishing region, cyclically changing said rate of agitation to shift the resulting potential difference-current density values to a point outside said polishing region for a period of time short enough to avoid the formation of a permanent etch or a permanent coloration, and repeating said cycle untilat least a portion of the anode acquires a polish.

10. Method for the anodic polishing of brass in anelectrolyticcell containing a cyanide bath in which' the rate of agitation of the anode with respect to the electrolyte affects the potential difference-current density curve and the location of the equilibrium etching, polishing vand permanent coloration regions with respect to said curve, said method comprising the steps of setting up an initial agitation of such Ymetal with respect to the electrolyte, applying, apotential difference between the anode `and the cathode, said potential difference being in. one of said regions outside said equilibrium polishing region for a .period of time short enough'V to avoid the formation of an etch or a permanent coloration, cyclically changing said rate of agitation to shift the resulting potential differenceourrent density values to a point within said equilibrium polishing region and repeating said cycleauntil at least a portion of the anode acquires a polish. l

l1. Methodfor the anodic polishing of nickel silver in an electrolytic cell containing a cyanide bathin which the rate of agitation of the anode with respect yto the electrolyte affects the potential difference-current density curve and the location of the equilibrium etching, polishing and permanent coloration regions with respect to said curve, said method comprising the steps of setting up an initial agitation of such metal with respect to the electrolyte, applying a potential difference between the anode and the cathode, said potential difference being in the equilibrium polishing region, cyclically changing said rate of agitation to shift the resulting potential difference-current density values to a point outside said polishing region for a period of time short enough to avoid the formation of a permanent etch or a permanent coloration, and repeating said cycle until at least a portion of the anode acquires a polish.` l'

12. Method for the anodic polishing of'nickel silver in an electrolytic cell containing acyanide bath in which the rate of agitation of the anode with respect to the electrolyte affects theV potential diiference-current density curve` and the location of the equilibrium etching, polishing and permanent coloration regions with respect t0 said curve, said method comprising the steps of setting up an initial agitation of such' metal with respect to the electrolyte, applying a potential difference between the anode andthe cathode, said potential difference being'in one of said regions outside said equilibrium polishing region for a period of time short enough to avoid the formation of an etch or a Vpermanent coloration, cyclically `changing said rate of agitation to shift the resulting potential differencecurrent density values to a point within said equilibrium polishing region and repeatingsaid cycle until at least a` portion of the anode ac-` quires a polish. h

SAMUEL EATON'.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS' Number Name Date 1,566,255 AltSelI Dec. 22, 1925 2,416,294 Eaton Feb. 25, 1947 2,473,923 Turner June 21; 1949 OTHER REFERENCES Transactions of The Electrochemical Society, vol. 8l (1942), pages 199-204. 

1. METHOD FOR THE ANODIC POLISHING OF SILVER, COPPER, CADMIUM, ALLOYS CONSISTING THE THESE METALS, BRASS, ZINC AND NICKEL SILVER IN AN ELECTROLYTIC CELL CONTAINING A CYANIDE BATH IN WHICH THE RATE OF AGITATION OF THE ANODE WITH RESPECT TO THE ELECTROLYTE AFFECTS THE POTENTIAL DIFFERENCE-CURRENT DENSITY CURVE AND THE LOCATION OF THE EQUILIBRIUM ETCHING, POLISHING AND PERMANENT COLORATION REGIONS WITH RESPECT TO SAID CURVE, SAID METHOD COMPRISING THE STEPS OF SETTING UP AN INITIAL AGITATION OF SUCH METAL WITH RESPECT TO THE ELECTROLYTE, APPLYING A POTENTIAL DIFFERENCE BETWEEN THE ANODE AND THE CATHODE, SAID POTENTIAL DIFFERENCE BEING IN THE EQUILIBRIUM POLISHING REGION, CYCLICALLY CHANGING SAID RATE OF AGITATION TO SHIFT THE RESULTING POTENTIAL DIFFERENCE-CURRENT DENSITY VALUES TO A POINT OUTSIDE SAID POLISHING REGION FOR A PERIOD OF TIME SHORT ENOUGH TO AVOID THE FORMATION OF A PERMANENT ETCH OR A PERMANENT COLORATION, AND REPEATING SAID CYCLE UNTIL AT LEAST A PORTION OF THE ANODE ACQUIRES A POLISH. 